This invention was developed with Government support under Government contracts F33615-01-2-2108 and F33615-00-C-5403. The Government may have certain rights in this invention. The present invention relates to increasing the quality and desired properties of semiconductor materials used in electronic devices, particularly power electronic devices. In particular, the invention relates to an improved process for minimizing crystal defects in silicon carbide, and the resulting improved structures and devices.
The term “semiconductor” refers to materials whose electronic properties fall between the characteristics of those materials such as metals that are referred to as conductors, and those through which almost no current can flow under any reasonable circumstances which are typically called insulators. Semiconductor materials are almost invariably solid materials and thus their use in electronic devices has led to the use of the term “solid state”, to generally describe electronic devices and circuits that are made from semiconductors rather than from earlier generations of technologies such as vacuum tubes.
Historically, silicon has been the dominant material used for semiconductor purposes. Silicon is relatively easy to grow into large single crystals and is suitable for many electronic devices. Other materials such as gallium arsenide have also become widely used for various semiconductor devices and applications. Nevertheless, silicon and gallium arsenide based semiconductors have particular limitations that generally prevent them from being used to produce certain types of devices, or devices that can be used under certain operating conditions. For example, the respective bandgaps of silicon and gallium arsenide are too small to support the generation of certain wavelengths of light in the visible or ultraviolet areas of the electromagnetic spectrum. Similarly, silicon and gallium arsenide based devices can rarely operate at temperatures above 200° C. This effectively limits their use as devices or sensors in high temperature applications such as high power electric motor controllers, high temperature combustion engines, and similar applications.
Accordingly, silicon carbide (SiC) has emerged over the last two decades as an appropriate candidate semiconductor material that offers a number of advantages over both silicon and gallium arsenide. In particular, silicon carbide has a wide bandgap, a high breakdown electric field, a high thermal conductivity, a high saturated electron drift velocity, and is physically extremely robust. In particular, silicon carbide has an extremely high melting point and is one of the hardest known materials in the world.
Because of its physical properties, however, silicon carbide is also relatively difficult to produce. Because silicon carbide can grow in many polytypes, it is difficult to grow into large single crystals. The high temperatures required to grow silicon carbide also make control of impurity levels (including doping) relatively difficult, and likewise raise difficulties in the production of thin films (e.g. epitaxial layers). Because of its hardness, the traditional steps of slicing and polishing semiconductor wafers are more difficult with silicon carbide. Similarly, its resistance to chemical attack and impurity diffusion makes it difficult to etch and process using conventional semiconductor fabrication techniques.
In particular, silicon carbide can form over 150 polytypes, many of which are separated by relatively small thermodynamic differences. As a result, growing single crystal substrates and high quality epitaxial layers (“epilayers”) in silicon carbide has been, and remains, a difficult task.
Nevertheless, based on a great deal of research and discovery in this particular field, including that carried out by the assignee of the present invention, a number of advances have been made in the growth of silicon carbide and its fabrication into useful devices. Accordingly, commercial devices are now available that incorporate silicon carbide to produce blue and green light emitting diodes, as a substrate for other useful semiconductors such as the Group III nitrides, for high-power radio frequency (RF) and microwave applications, and for other high-power, high-voltage applications.
As the success of silicon-carbide technology has increased the availability of certain SiC-based devices, particular aspects of those devices have become more apparent. In particular, it has been observed that the forward voltage (Vf) of some percentage of silicon carbide-based bipolar devices tends to increase noticeably after prolonged operation of those devices. In this regard, the term “bipolar” is used in its usual or customary sense to refer to any device in which operation is achieved at least partially by means of minority carrier injection such that conduction through some region of the device is accomplished using both electrons and holes as carriers simultaneously or a device in which, during forward conduction, there is at least one forward biased p-n junction. This substantial change in forward voltage represents a problem that can prohibit the full exploitation of silicon carbide-based bipolar devices in many applications. Although multiple defects may be responsible for the observed Vf degradation (also called Vf drift), present research indicates that one of the causes for the increase in forward voltage is the growth of planar defects such as stacking faults in the silicon carbide structure under the application of forward current in a bipolar device. Stated differently, the passage of electric current through a silicon carbide bipolar device tends to initiate or propagate (or both) changes in the crystal structure. As noted above, many SiC polytypes are in close thermodynamic proximity, and solid phase transformations are quite possible. When the stacking faults progress too extensively, they tend to cause the forward voltage to increase in an undesirable manner that can prevent the device from operating as precisely as required or desired in many applications. Other types of crystallographic defects can likewise cause degradation. The “Vf drift” degradation problem discussed above is a well known and serious concern for designers of SiC power devices.
As those familiar with crystal structure and growth are well aware, perfect crystal structures are never achieved. There are a number of fundamental reasons for such imperfections: all crystals vibrate and contain a finite number of thermodynamically stable structural defects (because the crystals exist above 0 K), all are generally subject to the effects of light or other electromagnetic radiation, all contain some (even if very few) impurities and all have an actual surface because they are finite in size. For these and other reasons, crystal flaws, including stacking faults, can be expected to appear even under the best of growth circumstances.
Accordingly, there is presently a need in the art for an improved silicon-carbide growth technique and resulting structure that minimizes or eliminates the problem of increasing forward voltage (Vf drift) caused by the propagation of faults during operation, as well as a method for forming silicon carbide-based bipolar devices that minimizes or eliminates the undesired electronic side effects of faults and their growth under the application of forward current.